the unix/xen interface for suspend/reusme

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

In Vincent's new UNIX model, we end up with a UNIX daemon that can accept 'OS 
requests' via a control socket.  The first set of inputs is primarily to do 
with device hotplug (discussed here in the tuntap discussion), but there is 
also another interesting one in Xen: suspend/resume and live migration.

Vincent: see this pull request from Jon to mirage-skeleton:
https://github.com/mirage/mirage-skeleton/pull/2/files

Suspend/resume is a little complicated in Xen since the application has to 
somehow also keep an eye on the xenstore notifications, and invoke callbacks to 
do something (e.g. send a gratuitous ARP on wakeup).

Jon's example code is quite intrusive and probably ought to be factored into 
the OS library somewhere so that applications can register interest in such 
system-level events more easily.  It might be easiest to test this interface in 
the UNIX version by transmitting an explicit suspend/resume across the control 
socket so that UNIX containers could be put to sleep just like in Xen...

-anil